Self-aligned semiconductor trench structures

ABSTRACT

Methods for forming a semiconductor device include forming self-aligned trenches, in which a first set of trenches is used to align a second set of trenches. Methods taught herein can be used as a pitch doubling technique, and may therefore enhance device integration. Further, employing a very thin CMP stop layer, and recessing surrounding materials by about an equal amount to the thickness of the CMP stop layer, provides improved planarity at the surface of the device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/725,384, filed Dec. 21, 2012, now U.S. Pat. No. 8,552,526, which is acontinuation of U.S. patent application Ser. No. 13/347,478, filed Jan.10, 2012, now U.S. Pat. No. 8,343,875, which is a divisional of U.S.patent application Ser. No. 12/209,117, filed Sep. 11, 2008, now U.S.Pat. No. 8,101,497, the disclosures of each of which are herebyincorporated herein by reference in their entireties.

FIELD OF INVENTION

Embodiments relate to methods for forming semiconductor structures, morespecifically to methods for forming self-aligned trenches insemiconductor processing.

BACKGROUND OF THE INVENTION

One way that integrated circuit designers make faster and smallerintegrated circuits is by reducing the separation distance between theindividual elements that comprise the integrated circuit. This processof increasing the density of circuit elements across a substrate istypically referred to as “scaling” or increasing the level of deviceintegration. In the process of designing integrated circuits with higherlevels of integration, improved device constructions and fabricationmethods have been developed.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention disclosed herein are illustratedin the accompanying drawings, which are not drawn to scale but are forillustrative purposes only:

FIG. 1 illustrates a cross-sectional view of a partially formedsemiconductor device.

FIG. 2 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 1 after a photo mask has been used topattern a hard mask.

FIG. 3 illustrates a cross-sectional view of the partially formedsemiconductor device of FIG. 2 after the hard mask has been used topattern a second hard mask.

FIG. 4 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 3 after certain layers have been patternedand spacers formed therein.

FIG. 5 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 4 after etching a first set of trenchesinto the substrate.

FIG. 6 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 5 after filling the trenches.

FIG. 7 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 6 after planarization.

FIG. 8 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 7 after selective etching of a placeholderlayer from between filled trenches and deposition of a spacer material.

FIG. 9 illustrates a cross-sectional View of the partially formedsemiconductor device of FIG. 8 after performing a spacer etch andsubsequently selectively etching a second set of trenches into thesubstrate in a self-aligned manner.

FIG. 10 illustrates a cross-sectional view of the partially formedsemiconductor device of FIG. 9 after the second set of trenches havebeen filled

FIG. 11 illustrates a cross-sectional view of the partially formedsemiconductor device of FIG. 10 after planarization stopping on an etchstop layer.

FIG. 12 illustrates a detail of a cross-sectional view of the partiallyformed semiconductor device of FIG. 11 after portions of the surfacehave been selectively recessed.

FIG. 13 illustrates a detail of a cross-sectional view of the partiallyformed semiconductor device of FIG. 12 after the etch stop layer hasbeen removed.

FIG. 14 shows a top-down view of the semiconductor device of FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Disclosed herein are fabrication techniques for semiconductor structuresincluding self-aligned trenches. Self-aligned trenches can be used aspart of pitch doubling techniques, which promote device integration. Thefabrication techniques described herein advantageously enable moreprecise etching, minimize stress to the substrate and/or promoteplanarity at the surface of the semiconductor structure.

FIG. 1 is a cross-sectional view of a partially formed semiconductordevice including a substrate 110 over which several additional layershave been formed to facilitate a masking process. The substrate 110comprises one or more of a wide variety of suitable workpieces forsemiconductor processing. In some embodiments, the substrate 110includes semiconductor structures that have been fabricated thereon,such as doped silicon platforms. While the illustrated substrate 110comprises a monocrystalline silicon wafer, in other embodiments thesubstrate 110 comprises other forms of semiconductor layers, whichoptionally include other active or operable portions of semiconductordevices. Substrate is also used herein to refer to the workpieceincluding integrated layers formed thereover.

As shown in FIG. 1, in some embodiments an oxide layer 210 is grown ordeposited on the substrate 110. The oxide layer 210 may comprise a thinpad oxide with a thickness between about 30 Å and 150 Å. The oxide layer210 can be deposited using a suitable deposition process, such aschemical vapor deposition (“CVD”) or physical vapor deposition (“PVD”),or is grown by oxidation of the underlying surface.

The partially formed semiconductor device of FIG. 1 also shows an etchstop 211 formed over the oxide layer 210 by a suitable depositionprocess such as CVD or PVD. The etch stop 211 serves as a stop layer fora later planarization step (e.g., a CMP stop layer), as will bedescribed hereinbelow. Relative to conventional CMP stop layers, theetch stop 211 can be very thin, e.g., may have a thickness between about20 Å and 150 Å, more particularly between about 50 Å and 100 Å. As willbe appreciated from the description below, this thin etch stop canimprove planarity for subsequent processing. In some embodiments, theetch stop 211 comprises nitride, such as silicon nitride (“Si₃N₄”). Inother embodiments, the etch stop 211 comprises aluminum oxide (“Al₂O₃”)or another material against which a neighboring material, particularlysilicon oxide, can be selectively etched by a chemical and/or mechanicaletching process.

As shown in FIG. 1, some embodiments also provide a placeholder material212 formed above the etch stop 211. Advantageously, the placeholdermaterial 212 may have a thickness sufficient to provide space for theformation of a spacer material in a later step, as describedhereinbelow. Thus, the placeholder material 212 may have a thicknesssuitable for later definition of a sidewall spacer height, such asbetween about 500 Å to 3000 Å, more particularly 800 Å to 1000 Å. In theillustrated embodiment, the placeholder material 212 comprisespolysilicon, although other materials that are selectively etchable withrespect to surrounding materials, and particularly trench isolationmaterials, can also be used.

In accordance with embodiments of the present invention, a hard mask isprovided for etching a first set of trenches. In the embodiment shown inFIG. 1, two hard mask layers 213, 214 are provided, the first hard masklayer 214 being used to pattern the second hard mask layer 213, and thesecond hard mask layer 213 serving as the mask through which the firstset of trenches will be etched. This embodiment is described in greaterdetail below, as this arrangement provides certain performanceadvantages, such as an improved aspect ratio for etching the first setof trenches. However, other embodiments are also possible wherein afirst set of trenches is etched using only a single hard mask, such asby using a photomask to etch a carbon hard mask and then etching thetrenches through the carbon hard mask. In still other embodiments, nohard mask is used at all and instead the first set of trenches aresimply formed using a photomask.

Referring again to the embodiment illustrated in FIG. 1, the second hardmask layer 213 is formed over the placeholder material 212 by anysuitable deposition process such as CVD or PVD. The second hard masklayer 213 may comprise a form of silicon oxide, such astetraethylorthosilicate (“TEOS”), and it may have a thickness betweenabout 500 Å to 1500 Å, more particularly 800 Å to 1200 Å. The first hardmask layer 214 can comprise carbon, such as amorphous carbon. Inparticular, the amorphous carbon can be a form of transparent carbonthat is highly transparent to light, such as is disclosed in A.Helmbold, D. Meissner, Thin Solid Films, 283 (1996), 196-203, the entiredisclosure of which is incorporated herein by reference. The first hardmask layer 214 may have a thickness between about 1000 Å to 3000 Å, moreparticularly 1800 Å to 2200 Å. Accordingly, the fraction of thethickness of the second hard mask 213 as a percentage of the thicknessof the first hard mask 214 is less than 100%, more particularly 30-70%,even more particularly 40-60%. This reduced thickness of second hardmask layer 213, as compared to the first hard mask layer 214,advantageously provides a lower aspect ratio when the first set oftrenches are etched as compared to using the first hard mask 214directly.

As shown in FIG. 1, some embodiments also provide coatings 215, 216formed above the first hard mask 214. A dielectric antireflectivecoating (“DARC”) 215 advantageously protects the first hard mask 214.The DARC 215 may comprise a silicon rich material, such as siliconoxynitride (“Si_(x)O_(y)N_(z)”). An organic bottom antireflectivecoating (“BARC”) 216 may also be included at the interface with aphotoresist 218.

The photoresist 218 shown in FIG. 1 provides the pattern for removingportions of the first hard mask 214 through photolithography and etchingtechniques. In some embodiments, as shown in FIG. 1, the photoresist 218provides a pattern of parallel lines (shown in cross-section in FIG. 1;see also FIG. 13) separated from each other by gaps. In someembodiments, such as when a high degree of device integration isdesired, the width of the lines and the gaps between the lines may beapproximately equal to “F”, where F can be the minimum feature sizeformable using a particular photolithographic technique. In general,however, “F” is used in the figures to represent relative dimensions.The skilled artisan will appreciate that the methods described hereincan be implemented where F is greater than the minimum resolution. Otherembodiments can utilize photoresists 218 defining other patterns, whichin turn produce different configurations of trenches in later steps. Inan exemplary embodiment, the photoresist 218 is formed by spinning on aphotosensitive material, exposing the photosensitive material to lightthrough a mask containing the desired pattern, and then developing thematerial into the photoresist 218. Next, the first hard mask 214 can beetched through the gaps in the photoresist 218. After thephotolithography and etching of the first hard mask 214 has been carriedout, the photoresist 218 can be stripped, or the photoresist can beremoved during the etching of the first hard mask 214, thus producingthe partially formed semiconductor device shown in FIG. 2.

In FIG. 2, a first hard mask 214 has been etched with a pattern ofalternating lines and gaps. As noted above, in some embodiments, thefirst set of trenches can be etched into the substrate through the firsthard mask 214. However, in the embodiment illustrated in FIGS. 2-3, therelatively thinner second hard mask 213 is etched through the thickerfirst hard mask 214, and the first hard mask 214 is then removed. Thisprocess produces the partially formed semiconductor device shown in FIG.3, in which the second hard mask 213 has been patterned.

With reference to FIG. 4, the pattern of the second hard mask 213 hasbeen transferred (e.g., via selective wet etch) to the placeholdermaterial 212. An upper sidewall spacer 217 can be formed along thesidewalls of the placeholder material 212 and the second hard mask 213.The upper sidewall spacer 217 can be formed by conformally depositing auniformly thick silicon oxide, such as the same material in the secondhard mask 213, over upper and sidewall surfaces, and then directionallyetching the deposited oxide such that horizontal surfaces arepreferentially removed but the upper sidewall spacer 217 remains in thedesired locations. In some embodiments, the upper sidewall spacers 217have a thickness of approximately ¼ F, which can provide a gap betweenthe upper sidewall spacers 217 of approximately ½ F. Such configurationscan advantageously provide two uniform sets of trenches withapproximately equal widths and separated by approximately equaldistances, as will be described hereinbelow.

With reference to FIG. 5, a first set of trenches 300 has been etchedinto the substrate 110 through the second hard mask 213 and the uppersidewall spacers 217. The etching process can be a selective etchingprocess that preferentially etches the material to be removed for thetrench, such as silicon, without etching the material of the second hardmask 213 and spacers 217, such as TEOS or other silicon oxide-basedmaterial. The skilled artisan will appreciate that other hard maskmaterials and other selective etch chemistries can be employed, such aswhen damascene trenches are being formed in an interlevel dielectric(“ILD”). Hence, FIG. 5 shows the second hard mask 213 still in placeafter the formation of the first set of trenches 300. Nonetheless, theskilled artisan will recognize that the selective etching may beimperfect, thus eliminating all or a portion of the second hard mask 213in the process of etching the first set of trenches 300.

In some embodiments, the trenches of the first set 300 are separated bya series of inter-trench regions 301, in which a second set of trenchescan later be formed. In some embodiments, the width of the inter-trenchregions 301 may be approximately equal to 1.5 F. Thus, in the embodimentillustrated in FIG. 5, after the trenches of the first set 300 have beenformed, the trenches have a width approximately equal to ½ F, the uppersidewall spacers 217 have a width approximately equal to ¼ F, and theinter-trench regions 301, which include the upper sidewall spacers 217,have a width approximately equal to 1.5 F.

The trenches of the first set 300 have a depth “D₁” defined by thedistance from the top of the substrate 110 in the inter-trench regions301 to the bottom silicon of the trenches 300 immediately after the stepof etching the first set of trenches 300. In some embodiments, D₁ isbetween about 2000 Å and 3000 Å, more particularly 2000 Å to 2500 Å.

The trenches of the first set 300 also have an aspect ratio defined bythe ratio between a depth “D2” of the trenches (in this case includingthe masking layers) and a width of the trenches at the top of thetrenches. The depth D2 is defined as the distance from the top surfacein the inter-trench regions 301 to the bottom of the trenches 300immediately after the step of etching the first set of trenches 300. Forreasons explained hereinabove, immediately after the step of etching thefirst set of trenches, the top surface of the inter-trench regions 301may be at the top of the second hard mask 213 (as shown in FIG. 5) or atthe top of the placeholder material 212. In some embodiments, the aspectratio is between about 5:1 and 100:1, more particularly between about10:1 and 25:1.

Following the formation of the first set of trenches 300, the trenchescan be filled. In some embodiments, the trenches 300 may be lined withone or more liner layers. For example, in the embodiment illustrated inFIG. 6, a thin oxide layer 220 is grown in the trenches as well as thesurface of the inter-trench regions 301 with a thickness of betweenabout 20 Å to 70 Å. The oxide layer 220 advantageously repairs etchdamage to the walls of the trenches 300. FIG. 6 also shows a nitridelayer 221 with a thickness between about 30 Å to 100 Å that has beendeposited above the oxide layer 220 as a barrier layer to facilitatelater oxide densification. A TEOS layer 223 of thickness between about100 Å to 300 Å may optionally be formed above the nitride layer 221,providing a diffusion path for more even distribution of oxidant duringSOD densification. The trenches 300 are then filled with a fillermaterial 225, such as an electrically isolating material. In anexemplary embodiment, filler material 225 comprises a spin-ondielectric. As shown in FIG. 6, the filling step may include overfillingthe trenches 300, such that the filler material 225 extends above thetop of the trenches 300.

FIG. 7 illustrates a cross-sectional view of the partially formedsemiconductor device of FIG. 6 after the surface has been planarized.The planarizing process may comprise a chemical mechanical polishingprocess. As shown in FIG. 7, the planarizing step may stop on theplaceholder material 212, such that after the planarizing step, thesurface of the partially formed device comprises the filler material 225in the trenches 300 and the exposed placeholder material 212 in theinter-trench regions 301.

Next, the upper sidewall spacers 217 can be removed (if they have notalready been removed during the earlier processes), and the placeholdermaterial 212 can be selectively etched from the inter-trench regions301, thus producing a partially formed device in which the fillermaterial 225 in the trenches 300 protrudes above the surface of theinter-trench regions 301. In some embodiments, TMAH can be used in anisotropic wet etch to selectively remove the placeholder material 212from the inter-trench regions 301. A second spacer material 230, such asTEOS, may then be blanket deposited to conform to the surface of thedevice, as shown in FIG. 8.

The spacer material 230 can then be preferentially etched fromhorizontal surfaces using a directional spacer etch. The directionalspacer etch leaves in place second spacer material 230 in the form ofsidewall spacers on approximately vertical surfaces, such as thesidewalls of the portions of the filler material above the trenches 300,protruding above the surface of the inter-trench regions 301. Theseremaining portions of the second spacer material 230 can then be used toalign a second set of trenches 302 within the inter-trench regions 301,spaced by the spacers from the first trenches 300. In some embodiments,following the preferential etch of the second spacer material 230, thespacer material can have a thickness approximately equal to ½ F. Suchembodiments can provide approximately uniform widths of the first andsecond sets of trenches as well as approximately uniform spacing or gapsbetween the first and second sets of trenches.

FIG. 9 shows a cross-sectional view of the partially formedsemiconductor device after the second set of trenches 302 has beenetched into the substrate 110. FIG. 8 shows an embodiment in which thefirst trenches 300 are deeper than the second trenches 302, but otherconfigurations are possible: the second set could be deeper than thefirst set, or the first set and the second set could be approximatelythe same depth. As noted above, embodiments can provide a technique forpitch doubling, wherein a trench of the first set 300 is spaced lessthan a distance F (e.g., approximately ½ F) from a trench of the secondset 302.

FIG. 10 shows the partially formed semiconductor device after the secondtrenches 302 have been filled. The second trenches 302 can be lined withan oxide liner 310, such as an oxide that is grown to a thicknessbetween about 30 Å and 150 Å. As shown in FIG. 10, the trenches 302 maythen be overfilled with a second filler material 312, such as with ahigh density plasma (“HDP”) oxide that extends above the top of thetrenches 302. Note that while the illustrated trenches 300, 302 havebeen filled in the illustrated embodiment by different techniques andembodiments, both sets of trenches are filled by forms of silicon oxidethat chemically similar from the point of view of selective etching.

Following the filling of the trenches 302, the partially formed devicemay then be planarized. The planarization may be carried out using achemical mechanical polishing (“CMP”) process, and it may stop on theetch stop 211. It is typically difficult to control the planarizationprocess with sufficient precision to stop at the top surface of the thinetch stop 211, such that a majority of the illustrated thin etch stop211 thickness is consumed at the end of the CMP process. Typicallybetween about 10 Å and 100 Å of the etch stop 211 is consumed instopping the CMP process, which can represent between about 50% and 80%of the etch stop 211 thickness. A cross-sectional view of a partiallyformed device after this planarization step has been carried out isillustrated in FIG. 11.

In some embodiments, the surface of the device after the planarizationstep may primarily comprise an etch stop 211 and structural material,such as silicon oxide. The structural material can then be selectivelyrecessed to approximately coplanar with the lower surface of the etchstop, for example by selectively etching the oxide against the nitrideof the etch stop.

FIG. 12 illustrates a detail of a device's surface after structuralmaterials have been selectively recessed. The amount that structural(illustrated as oxide) materials are recessed is preferably about equalto the thickness of the remaining etch stop 211. In the illustratedembodiment, the structural materials are recessed by between about 10 Åand 70 Å, more particularly between 30 Å and 50 Å. Thus, the recessedstructural materials leave an upper surface approximately (e.g., within±20 Å, more particularly within ±10 Å) coplanar with the lower surfaceof the etch stop 211. As shown in FIG. 12, other portions of the device,such as nitride liner layer 221, may protrude above the selectivelyrecessed portion along with the etch stop 211.

Next, the etch stop 211 can be removed, for example by selectivelyetching nitride. Such an etching step may also remove other portionsthat protrude above the surface of the structural material, such as thenitride liner layer 221. The inventors have observed that such a processenhances the planarity of the device's surface. FIG. 13 illustrates adetail of a semiconductor device after these steps.

FIGS. 11 and 14 illustrate parallel trenches filled with isolationmaterial, made in accordance with the methods described herein. In oneembodiment, the deeper trenches 300 represent shallow trench isolationbetween transistors in an array (e.g., a memory array such as DRAM).These deeper trenches alternate with parallel shallow trenches 302. Aswill be understood from the process flow of U.S. patent application No.2006-0046407, published Mar. 2, 2006, the disclosure of which isincorporated by reference herein, these shallow trenches 302 canrepresent the gap between source and drain pillars in a U-shapedprotruding transistor structure, and can remain filled with insulatingmaterial or can be replaced with gate dielectric and gate electrodematerials for a “3-sided” channel structure. As described in theincorporated '407 publication, subsequent crossing trenches of anintermediate depth can provide gaps between rows of transistors in whichgate electrode material can be formed to surround the U-shapedsemiconductor structures at least on two sides. In other embodiments,the trenches can be employed for deep DRAM capacitors; can all be fortrench isolation, and may be of approximately equal depth; can bedamascene trenches in insulating material for later filling with metallines; etc.

Although not illustrated in the figures, a sacrificial oxide layer witha thickness of between about 100 Å and 200 Å may then be grown andsubsequently stripped in order to remove and/or repair any damagedsilicon at the upper surface.

Methods have been described that provide several advantages for theformation of semiconductor structures. For example, methods have beentaught for using a placeholder material as part of the formation ofself-aligned trenches in substrates. Self-aligned trenches can be usedas part of pitch doubling techniques, which promote device integration.For example, in the illustrated embodiment, the pitch of the partiallyformed device was ½ F at the stage shown in FIGS. 1-7, whereas the pitchis F at the stage shown in FIG. 13; the feature size shrinks from F atthe stage of FIG. 1 to ½ F at the stage of FIG. 13. Fabricationtechniques described herein advantageously enable more precise etching,such as by using a relatively thin second hard mask to improve theaspect ratio during etching of the trenches. Further, methods disclosedherein promote planarity at the surface of the semiconductor structure,such as by providing a relatively thin etch stop, and a sequence ofrecessing surrounding materials by an amount about equal the amount ofetch stop remaining after consumption during a CMP step.

In accordance with one embodiment, a method is provided for forming adevice. The method comprises forming a layer of polysilicon on asubstrate and forming a first set of trenches in the substrate, whereinremaining portions of the polysilicon layer remain above the substratein inter-trench regions between trenches of the first set. The methodfurther comprises filling the first set of trenches with a fillermaterial, wherein the filler material extends upward to at least a leveladjacent the remaining portions of the polysilicon layer. Further, themethod comprises selectively etching the remaining portions of thepolysilicon layer from the inter-trench regions, forming spacers onsidewalls of the filler material in the inter-trench regions, andetching a second set of trenches into the substrate between the spacers.

In another embodiment, a method is provided for forming a device on asubstrate. The method comprises forming an etch stop layer on thesubstrate and etching a plurality of first trenches through the etchstop layer and the substrate. The method further comprises filling thefirst trenches with an isolation material that protrudes above thesubstrate, forming spacers on sidewalls of the protruding portion of theisolation material, and etching a plurality of second trenches betweenthe spacers. The method further comprises filling the second trencheswith a second filler material, planarizing the second filler materialand stopping planarizing on the etch stop layer.

In another embodiment, a method is provided of forming an integratedcircuit. The method comprises forming a structure on a substrate, thestructure comprising a structural material and an etch stop, the etchstop having an upper surface and a lower surface. The method furthercomprises planarizing the structure and stopping planarizing on the etchstop. The method further comprises selectively recessing the structuralmaterial to be approximately coplanar with the lower surface of the etchstop, and selectively removing the etch stop.

It will be appreciated by those skilled in the art that various otheromissions, additions and modifications may be made to the methods andstructures described above without departing from the scope of theinvention. For example, while the illustrated embodiments involveetching alternating “shallow” trench isolation (“STI”) and in asemiconductor material and filling with electrically insulatingisolation materials, the skilled artisan will appreciate applications ofthe principles and advantages taught herein for other contexts. Forexample, some of the methods taught herein can be applied to definingclosely spaced, self-aligned damascene trenches for filling with metallines. All such changes are intended to fall within the scope of theinvention, as defined by the appended claims.

What is claimed is:
 1. A method of forming a device, comprising:providing a substrate; forming a plurality of first trenches in asubstrate, comprising: forming a plurality of placeholder linesextending in a first direction; and etching the first trenches through asurface exposed between the placeholder lines; filling the firsttrenches with an isolation material; forming a plurality of secondtrenches in the substrate, comprising: selectively removing theplaceholder lines to form isolation lines comprising the isolationmaterial, the isolation lines extending in the first direction andprotruding above the surface; forming sidewall spacers on sidewalls ofthe isolation lines; and etching the second trenches through the surfaceexposed between the sidewall spacers of adjacent isolation lines.
 2. Themethod of claim 1, wherein forming the placeholder lines comprisesdepositing a layer of placeholder material and lithographicallypatterning.
 3. The method of claim 2, wherein the layer of placeholdermaterial comprises polysilicon.
 4. The method of claim 2, whereinforming the placeholder lines further comprises depositing a hardmaskmaterial on the placeholder material and lithographically patterning toform hardmask lines over the placeholder material.
 5. The method ofclaim 2, further comprising chemical-mechanically polishing afterfilling the first trenches to form a substantially planar surface,wherein the substantially planar surface comprises alternatingplaceholder lines and isolation lines.
 6. The method of claim 2, furthercomprising providing an etch stop material interposed between thesubstrate and the placeholder material.
 7. The method of claim 6,wherein the first trenches extend below the surface at a first trenchdepth and the second trenches extend below the surface at a second depthshallower than the first depth.
 8. The method of claim 1, whereinforming the first trenches further comprises conformally depositing aninitial sidewall spacer material over the placeholder lines anddirectionally etching the initial sidewall spacer material such that theinitial sidewall spacer material remains over sidewalls of theplaceholder lines.
 9. The method of claim 8, wherein forming theplaceholder lines further comprises forming lines of hardmask materialover the placeholder material, and wherein the initial sidewall spacermaterial and hardmask material comprise the same material.
 10. Themethod of claim 1, wherein filling the first trenches comprises liningthe first trenches with a first liner after etching the first trenches.11. The method of claim 10, further comprising lining the secondtrenches with a second liner comprising a second liner material afteretching the second trenches, wherein the first liner comprises a firstliner material different from the second liner material.
 12. A method offorming a device, comprising: forming a plurality of placeholder lineson an etch stop layer; etching a first set of trenches through the etchstop layer exposed between the placeholder lines; filling the first setof trenches between the placeholder lines with an isolation material;selectively removing the placeholder lines to form isolation linescomprising the isolation material; forming pairs of sidewall spacers onsidewalls of the isolation lines; and etching a second set of trenchesthrough the etch stop layer exposed between the isolation lines.
 13. Themethod of claim 12, wherein the etch stop layer comprises siliconnitride.
 14. The method of claim 12, wherein the etch stop layercomprises aluminum oxide.
 15. The method of claim 12, wherein formingthe sidewall spacers comprises: conformally depositing a sidewall spacermaterial over the isolation lines; and directionally etching thesidewall spacer material, such that the sidewall spacer material remainsover sidewalls of the isolation lines while the sidewall spacer materialis removed between the isolation lines.
 16. The method of claim 12,wherein filling the first set of trenches comprises lining the first setof trenches with a first liner comprising a plurality of layers afteretching the first trenches; and further comprising lining the second setof trenches with a second liner after etching the second trenches. 17.The method of claim 16, wherein the second liner comprises an oxidelayer.
 18. The method of claim 17, wherein the first liner comprise abarrier layer.
 19. The method of claim 18, wherein the barrier layercomprises silicon nitride.
 20. The method of claim 16, wherein the firstliner comprises a silicon oxide layer and a silicon nitride layer.